Finely chop 2 medium onions and 2 cloves garlic. Cut 100 g mixed bacon into small cubes.
Heat 20 g butter in a large pot over medium heat. Add bacon and cook for 3 minutes until starting to crisp. Add onions and garlic, sauté for 5 minutes until translucent.
Add the canned tomatoes with their liquid, salt, and pepper to the pot. Simmer over medium-low heat for 30 minutes.
While the sauce simmers: peel 400 g jacket potatoes and cut into slices. Wash 1 kg leek and cut into finger-length pieces.
Bring a pot of salted water to a boil over medium-high heat. Add leek pieces and boil for 5 minutes. Drain, rinse in cold water, and drain thoroughly.
Cut 250 g smoked garlic sausage into slices.
Drain 500 g low-fat quark in a colander. Transfer to a bowl and mix with half of the grated Gouda cheese, 4 egg yolks, and 125 ml sweet cream. Season with salt and pepper.
Preheat your oven to 220°C (428°F). Lightly grease a baking dish.
Layer half the potato slices in the prepared baking dish. Spread half the tomato sauce on top, then half the quark mixture.
Layer the leek pieces on top. Spread the remaining tomato sauce over them and arrange the sausage slices on top. Spread the remaining quark mixture on top.
Sprinkle the remaining grated cheese over everything. Bake for 35 minutes at 220°C (428°F) until golden.